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Original Article

^a Istituto di Cibernetica e Biofisica, Consiglio Nazionale delle Ricerche, 16149 Genova, Italy
^b Institut für Biologische Informationsverarbeitung, 52425 Jülich, Germany
^c Biophysics Sector, International School for Advanced Studies, 34014 Trieste, Italy
Biophysics Sector, International School for Advanced Studies, Via Beirut 2, 34014 Trieste, Italy.39-040-2240470

menini{at}sissa.it

Upon stimulation by odorants, Ca²⁺ and Na⁺ enter the cilia of olfactory sensory neurons through channels directly gated by cAMP. Cyclic nucleotide–gated channels have been found in a variety of cells and extensively investigated in the past few years. Glutamate residues at position 363 of the subunit of the bovine retinal rod channel have previously been shown to constitute a cation-binding site important for blockage by external divalent cations and to control single-channel properties. It has therefore been assumed, but not proven, that glutamate residues at the corresponding position of the other cyclic nucleotide–gated channels play a similar role. We studied the corresponding glutamate (E340) of the subunit of the bovine olfactory channel to determine its role in channel gating and in permeation and blockage by Ca²⁺ and Mg²⁺. E340 was mutated into either an aspartate, glycine, glutamine, or asparagine residue and properties of mutant channels expressed in Xenopus laevis oocytes were measured in excised patches. By single-channel recordings, we demonstrated that the open probabilities in the presence of cGMP or cAMP were decreased by the mutations, with a larger decrease observed on gating by cAMP. Moreover, we observed that the mutant E340N presented two conductance levels. We found that both external Ca²⁺ and Mg²⁺ powerfully blocked the current in wild-type and E340D mutants, whereas their blockage efficacy was drastically reduced when the glutamate charge was neutralized. The inward current carried by external Ca²⁺ relative to Na⁺ was larger in the E340G mutant compared with wild-type channels. In conclusion, we have confirmed that the residue at position E340 of the bovine olfactory CNG channel is in the pore region, controls permeation and blockage by external Ca²⁺ and Mg²⁺, and affects channel gating by cAMP more than by cGMP.

Key Words: cyclic guanosine monophosphate • cyclic adenosine monophosphate • olfactory sensory neurons • site-directed mutagenesis

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